In commonly owned U.S. Pat. No. 3,760,106, for example, there has been disclosed a PCM telecommunication system including a memory for the storage of an activity pattern from a multiplicity of associated lines, this pattern being periodically updated with the aid of respective monitoring units for the control of a gate which blocks communication between a multiplexer and a line found to be inactive. In the conventional sampling of telephone and other voice-communication lines, activity is generally determined from the existence of a certain minimum voltage level persisting for a number of sampling cycles. These signal characteristics are ascertained with the aid of threshold and integrating circuits responding, for example, to a signal level of -40 dBm. Thus, the system discriminates against transients and low-level background noise.
Such a high degree of selectivity, however, tends to suppress voice signals of relatively low energy content which nevertheless are significant elements of human speech, such as fricative or sibilant sounds introducing a syllable of a spoken word. Failure to recognize these sounds as useful signals results in a loss of information or a distortion of meaning, as when an initial "s" is lost to make "sour" sound like "our". This drawback cannot be eliminated in conventional systems by a lowering of the sensitivity threshold since that would also increase the noise level.